The invention relates to the field of large antenna/receiver arrays in which output signals of the individual receivers are to be phase adjusted so that they can be coherently added, thereby producing an output signal having a greatly improved signal-to-noise ratio over that of any of the individual receiver output signals.
The concept of phase adjusting signals derived from a plurality of antenna/receiver arrays so that the signals can be coherently added is not new. One such system is described in an article entitled "Analysis of the Signal Combiner for Multiple Antenna Arraying" by R. A. Winkelstein, JPL Deep Space Network Progress Report 42-26, Pages 102-118, dated Apr. 15, 1975. However, in order for this system to perform properly, one of the receiver outputs or baseband signals must have a sufficiently high signal-to-noise ratio to serve as a reference for correlation with the other baseband signals so that their phases can be adjusted accordingly. If all receiver baseband signal-to-noise ratios are below a certain minimum level, then individual loop correlations for phase adjusting the baseband signals take an excessive amount of time, thereby seriously degrading alignment accuracy. Thus, conventional signal combiners are unable to satisfactorily provide a composite signal when there is a lack of a single strong signal to act as a phase reference for the remaining signal control loops. The present invention solves this problem by providing a means whereby all of the signals can be phase-correlated with one of the selected signals even though none of the individual signals has a sufficiently high signal-to-noise ratio to act as a reference for the control loops of the remaining signals. It has been found that in a typical system using a plurality of loop correlators to phase adjust the individual signals, one of the signals should have a signal-to-noise ratio greater than 0 dB. However, a system according to the present invention utilizing a 10 antenna array has been shown to operate with individual baseband signal-to-noise ratios as low as -7.7 dB.